1. Field of the Invention
The present invention relates to an inverter device used in a power converter or the like and, more specifically, to a method and device for controlling an inverter performing feedback control of an output waveform.
2. Description of the Background Art
An inverter control device shown in FIG. 1 has been known. The inverter control device 50 controls an inverter main circuit 53 which drives an AC load 52 using a DC power source 51 as an input. A DSP (Digital Signal Processor) 54 instantaneously controls the value of the output current from inverter main circuit 53.
The method of control by inverter control device 50 will be described in the following. A current detector 55 detects a value of an output current from inverter main circuit 53, and the output current value is subjected to A/D conversion at a prescribed sampling frequency by an A/D converter 56. The output current signal I.sub.0 provided by the A/D conversion is input together with a reference current signal I.sub.C to DSP 54. DSP 54 has functions of an error amplifying portion 57 and a PWM (pulse width modulation) operating portion 58, and based on the output current signal I.sub.0 and the reference current signal I.sub.C input to DSP 54, error amplifying portion 57 calculates an error amplified signal E=.alpha. (I.sub.C -I.sub.0). Based on the error amplified signal E, PWM operating portion 58 calculates a gate on time T.sub.on of a plurality of switching elements, not shown, constituting inverter main circuit 53. DSP 54 outputs the calculated gate on time T.sub.on to a timer count circuit 59. Based on the input gate on signal T.sub.on, timer count circuit 59 generates a gate driving signal G for each switching element, and outputs it to a gate driving circuit 60. In gate driving circuit 60, based on the gate driving signal G input thereto, switching of the switching elements in inverter main circuit 53 is performed.
As for the input of output current signal I.sub.0 and reference current signal I.sub.C, calculation of error amplified signal E, and calculation and output of gate on time T.sub.on, these are continuously executed at prescribed sampling period. Further, in order to suppress high frequency oscillation of the output current, a lowpass filter 61 may be provided on the side of the output signal of error amplifying portion 57. In this case, lowpass filter 61 filters the error amplified signal E, and PWM operating portion 58 performs PWM operation using the filtered output. In controlling an inverter, a delay generated in a signal path causes oscillation in a high frequency range. Therefore, in order to enable stable control of an inverter output, control of the delay is important. However, in the conventional inverter control device 50 in which the value is instantaneously controlled, generation of a delay in a feedback loop through a lowpass filter, not shown, included in inverter main circuit 53, current detector 55 for detecting the current value from the output of the inverter and through the DSP 54 calculating and outputting the gate on time T.sub.on is inevitable because of the structural characteristics.
In order to cope with this problem, conventionally, a lowpass filter is provided for reducing the influence of delay in the high frequency range, where the influence of delay is more severe, so as to lower the feedback gain in the high frequency range. However, even when such a measure is taken, there is still a delay generated in the lowpass filter itself. Therefore, the lowpass filter, which is provided as a solving means, causes a new delay, and high frequency oscillation cannot always be satisfactorily suppressed.
In order to lower feedback gain in the high frequency range effectively by using a lowpass filter, generally, cut off frequency is made lower or an amount of attenuation is increased. However, by such a method, the delay in the lowpass filter is increased, and therefore, the feedback gain cannot be readily reduced.